Photolithography has become a critical enabling technology in the fabrication of modern integrated circuit (IC) devices. The photolithography process typically involves exposing a patterned mask to radiation to produce patterned radiation. The patterned radiation is then passed through an optical reduction system, and the reduced patterned radiation or mask image is projected onto a substrate, typically a silicon wafer, that is coated with photoresist. The radiation exposure changes the properties of the photoresist and allows subsequent processing of the substrate.
Photolithography machines or steppers, use two common methods of projecting a mask image onto a substrate, "step and repeat" and "step and scan". The step and repeat method sequentially exposes portions of a substrate to a mask image and uses a projection field which is large enough to project the entire mask image onto the substrate. After each image exposure, the substrate is repositioned and the process is repeated.
Step and scan optical systems typically utilize ring fields to project the mask image onto the substrate, and are more commonly used in IC fabrication processes. The step and scan method scans a mask image onto a substrate through a narrow arc-shaped slit of light. Referring to FIG. 1, a ring-field lithography system 100 for use in the step and scan method is shown. A moving mask 101 is illuminated by a radiation beam 103, which reflects from the mask 101 and is directed through a reduction ring-field optical system 107. Within the optical system 107 the image is reversed and an arc shaped reduced image beam 109 is projected onto a moving substrate 111. The arc-shaped reduced image beam 109 can only project a portion of the mask 101 at one time, and is thus moved over time to scan the complete mask 101 onto the substrate 111. The mask 101 and substrate 111 are moved synchronously in relation to one another to perform the scanning process.
FIG. 2 is a more detailed illustration of the arcuate ring field slit 201 produced by the photolithography system illustrated in FIG. 1. Ring field arc 201 corresponds to the projection of the arc-shaped reduced image beam 109 in FIG. 1, as it is seen on the surface of substrate 111. Ring field arc 201 is projected over angle 209 and is geometrically described by a ring field radius 203, a ring field width 205, and a ring field length 207. In general, ring field coverage is unlimited in azimuth.
As the degree of circuit integration has increased, the feature sizes of IC's have dramatically decreased. To support future semiconductor fabrication requirements, lithography systems using extremely short wavelength radiation have been developed to overcome the inherent limitations of traditional optical systems. Extreme Ultraviolet (EUV) lithography using 10 to 14 nm soft x-ray photons has emerged as a promising technology for fabrication of integrated circuits with design rules requiring sizes of 100 nm or less. Deep Ultraviolet (DUV) lithography using 100-300 nm radiation has also proven to be a viable technology for IC fabrication lithography systems.
In lithography, where it is crucial that the image characteristics are invariant across the imaging field, the condenser optical system is a critical component. Because refractive optics absorb all EUV radiation, only reflective optical elements are suitable for EUV optical systems. In EUV lithography, therefore, the condenser is likely to be all-reflective and must gather as much light as possible from the source, in order to reduce exposure time to an economical level.
Most projection optics image over ring fields, whereas most EUV sources are small and circular. In order to make the most efficient use of the EUV source, a ring-field system requires a condenser that illuminates only the required arc of the ring field. The angular distribution of the illumination at any field point must have a particular profile (for example, a uniform circular disc) and must be directed into the center of the entrance pupil of the projection optical system.
The illumination system for a ring-field projection camera must satisfy a number of criteria. These criteria are driven by the lithographic requirements of the camera, namely, critical dimension (CD) uniformity and minimal pattern placement error across the field, the ability to image at high contrast, and the ability to print at a high rate. To satisfy these requirements, the condenser used in such an optical system must satisfy minimum performance characteristics, such as, shape and uniformity of the pupil fill; lack of any variation of illumination characteristics across the field (stationarity); telecentricity; EUV throughput; and compensation of camera errors. Of these, the pupil fill and stationarity are most important, since they encompass the main purpose of an illuminator which is to provide illumination of the mask in a way that optimizes its aerial image by scanning and minimizes any variation of CD across the field. Furthermore, the EUV throughput must be maximized due to the economic need to minimize exposure times. In addition to these optical metrics, there are also those of the scalability to higher numerical aperture (NA) imaging systems, and the ease of manufacturability of the condenser.
Present, known condenser designs have proven unable to meet all of the requirements demanded by EUV and DUV lithography in which the condenser must collect as much light as possible from a source such as a laser-produced plasma, discharge, or synchrotron source, and transform it into an arc at the mask plane with the desired pupil fill at the pupil plane of the projection optics. Present EUV lithography condensers exhibit certain critical drawbacks, including inefficiency due to the inability to use a large proportion of the EUV light, and non-uniformity of the pupil fill. These drawbacks have led to condensers that are unable to provide both the necessary arc illumination and angular distribution, thus leading to changes in imaging quality across the field and for different object orientation.